Standard cryogenic air separation processes involve filtering of feed air to remove particulate matter followed by compression of the air to supply energy for separation. Generally the feed air stream is then cooled and passed through absorbents to remove contaminants such as carbon dioxide and water vapor. The resulting stream is subjected to cryogenic distillation.
Cryogenic distillation or air separation includes feeding the high pressure air into one or more separation columns which are operated at cryogenic temperatures whereby the air components including oxygen, nitrogen, argon, and the rare gases can be separated by distillation.
Cryogenic separation processes involving vapor and liquid contact depend on the differences in vapor pressure for the respective components. The component having the higher vapor pressure, meaning that it is more volatile or lower boiling, has a tendency to concentrate in the vapor phase. The component having the lower vapor pressure meaning that it is less volatile or higher boiling tends to concentrate in the liquid phase.
The separation process in which there is heating of a liquid mixture to concentrate the volatile components in the vapor phase and the less volatile components in the liquid phase defines distillation. Partial condensation is a separation process in which a vapor mixture is cooled to concentrate the volatile component or components in the vapor phase and at the same time concentrate the less volatile component or components in the liquid phase.
A process which combines successive partial vaporizations and condensations involving countercurrent treatment of the vapor in liquid phases is called rectification or sometimes called continuous distillation. The countercurrent contacting of the vapor and liquid phases is adiabatic and can include integral or differential contact between the phases.
Apparatus used to achieve separation processes utilizing the principles of rectification to separate mixtures are often called rectification columns, distillation columns, or fractionation columns.
When used herein and in the claims, the term "column" designates a distillation or fractionation column or zone. It can also be described as a contacting column or zone wherein liquid or vapor phases are countercurrently contacted for purposes of separating a fluid mixture. By way of example this would include contacting of the vapor and liquid phases on a series of vertically spaced trays or plates which are often perforated and corrugated and which extend crosswise of the column, perpendicular to the central axis. In place of the trays or plates there can be used packing elements to fill the column.
"Double column" as used herein refers to a higher pressure column having its upper end in heat exchange relation with the lower end of a lower pressure column.
The term "a standard air separation process or apparatus" as used herein is meant to describe that process and apparatus as above described as well as other air separation processes well known to those skilled in the art.
As used herein and in the appended claims, the term "indirect heat exchange" means the bringing of two fluid streams into heat exchange relation without any physical contact or intermixing of the fluids with each other.
Historically, nitrogen, oxygen and/or argon have been produced by one of two basic process schemes including the single column process and the double column process.
With respect to nitrogen, the single column process produces good quality gaseous and liquid nitrogen at pressures of approximately 6-10 bar. The recovery of nitrogen is limited by the equilibrium at the bottom of the column. Typically, the process can produce nitrogen at a rate of approximately 50-60% of the nitrogen in the initial air feed.
With the double column process, nitrogen is produced at pressures of about 1-4 bar. It is more efficient than the single column process, and approximately 90% or more of nitrogen can be recovered from the nitrogen present in the initial air feed. Typically the columns are stacked with a condenser-reboiler separating the two columns. Since the process produces nitrogen at relatively low pressures, further compression of nitrogen is frequently needed adding to the cost of production and use.
In the prior art double column process, air is separated by cryogenic distillation or rectification to produce a nitrogen-rich stream or fraction at the top of the high pressure column and oxygen-rich stream or fraction at the bottom. The nitrogen-rich stream is sent to the top of the low pressure column to provide the reflux for this column. The bottom oxygen-rich stream is fed to the low pressure column for further separation.
In the low pressure column the feed stream is further separated by cryogenic distillation into an oxygen-rich stream or fraction at the bottom and a nitrogen-rich stream or fraction at the top. The top stream can then be recovered as nitrogen product. In the double column arrangement, the high pressure column and the low pressure column are thermally linked through the condenser-reboiler arrangement. Thus, in the prior art double column process the nitrogen-rich fraction of the high pressure column is condensed against the vaporizing oxygen-rich fraction of the low pressure column.
For a given pressure in the low pressure column, the pressure of the air feed to the high pressure column is dictated by the composition of the vaporizing oxygen-enriched stream, the temperature difference of the high pressure column condenser and the low pressure column reboiler, and to some extent the composition of the condensing nitrogen-enriched stream which is relatively pure in nitrogen.
Other prior art process schemes are variations of the above described single or double column process with additional features such as an additional overhead condenser or bottom reboiler.